I have a 92 3500 with a 7.4 acting the same way.
New IACV, EGR valve good, EGR vacuum solenoid good.
I started chasing this down two years ago. I did: rotor, cap, wires, gapped the plugs. It got a little better but still has problems.
It's acting like it's running rich to the point of flooding. Cold start is ok, runs great for a few minutes. Then you can hear the ECM cycling the IACV, it opens then closes a little and the engine idles better, then rough, it cycles like this until it stalls.
I haven't replaced the injectors yet. Could they be bad? The manuals say check the charcoal canister. But when I pull the line from the canister it doesn't change behavior, no difference if I plug the line or leave it open.
When I pulled the PCV hose from the throttle body, it idled fine, a little high but it ran ok. So I bought $10 of tube and fittings and put a little breather tee-d into that line, to allow some fresh air into the line along with gases from the PCV. It now idles when not in gear (was stalling even in park or neutral) but still stalls in gear.
Very frustrated. Bad injectors?